1. Field of the Invention
The invention relates to the field of electronic thermostats and more particularly to a system and method for detecting the status of a thermostat system, or output, relay to determine if the relay has been set to a requested state and, if not, providing a boosted or increased voltage to the relay in an attempt to set the relay to the correct state.
2. Description of Related Art
Electronic thermostats are well known in the art. Such thermostats include a system or output relay, typically a latching relay, that may be set to an ON or OFF status. The output relay includes output relay on/off contacts that are part of the thermostat switching circuitry that provides electrical control signals to heating and air conditioning systems to actuate the heating and air conditioning units in a building environment. That is, when the thermostat output relay is ON, signals are provided to enable the turning on of either a heating system or an air conditioning system in accordance with the user selecting the heating or cooling mode. Similarly, when the system relay is OFF, thermostat output signals are discontinued and the heating and/or cooling systems are turned off.
As is known in the art, the thermostat system output relay may be turned on or off by signals from a processing unit such as a microprocessor in accordance with user inputs and operating conditions of the environment served by the thermostat. That is, the microprocessor detects when a user has selected the turning on or off of the thermostat and initiates actuation of the thermostat output relay in accordance with the user's selection.
Systems are known to monitor the status of the thermostat output relay to determine if the relay is set at the correct state, i.e., the state as requested by the microprocessor. One such system is described in U.S. Pat. No. 5,277,244, inventor V. Mehta, assigned to the same assignee as the present invention, which recognizes that the system output relay includes contacts that may incorrectly respond to spurious signals, such as power line signals or even radio signals, to cause the relay contacts to change between its on and off states. In an effort to overcome this problem, the '244 patent provides for monitoring the status of the system or output relay and, if the status is different from, i.e., inconsistent with, the correct status as had been previously set or requested by the microprocessor, the microprocessor activates the relay to change the state of the relay consistent with the requested state.
It is a problem with the prior art system, however, that sometimes the state of the output relay is not correctly set because the voltage applied to the relay coil to set the relay is insufficient to enable activation of the relay coil. For example, if the thermostat battery voltage is too low, i.e., is at a level insufficient to be able to actuate the relay, then the state of the relay will not be changed. Thus, in using the system of the '244 patent, if the state of the output relay is incorrect resulting from a low battery condition, then the relay will not be correctly reset and the microprocessor's attempts to repeatedly provide this insufficient power to the relay coil will be unavailing to change the output relay contact state.